Laminate for electronic circuit

ABSTRACT

The present invention is directed to a laminate having a layer construction of metal-insulating layer-metal or a layer construction of metal-insulating layer, which laminate meets conditions for developing large adhesive strength between the insulating layer and the metal, as well as to an insulating film and an electronic circuit using the laminate. The laminate has a layer construction of first metal layer/insulating layer/second metal layer or a layer construction of metal layer/insulating layer. The insulating layer  1  has a multilayer structure of two or more layers. The layers on the side of the adhesive interface with each metal layer (a copper foil  3  and an SUS foil  4 ), out of the layers constituting the insulating layer  1 , each are a thermoplastic resin layer  2 . The minimum value of the storage modulus at a temperature at or above Tg of the thermoplastic resin layer  2  is not more than 10 6  Pa.

TECHNICAL FIELD

[0001] The present invention relates to a laminate having a layerconstruction of metal-insulating layer-metal and/or a layer constructionof metal-insulating layer, and a film consisting of only an insulatinglayer having a multilayer structure of two or more layers, and asubstrate for use mainly in the formation of a circuit on an electroniccomponent, particularly on an insulating layer, by taking advantage ofthe insulating properties of the laminate.

BACKGROUND ART

[0002] In recent years, rapid development of semiconductor technologyhas led to rapid progress of a reduction in size of semiconductorpackages, the adoption of multipin, the adoption of fine pitch,minimization of electronic components and the like. That is, thesemiconductor field has entered the so-called “age of high densitypackaging.” Regarding printed wiring boards, this has also led to achange from single side wiring to double side wiring and, in addition,the adoption of a multilayer structure and a thickness reduction (Iwataand Harazono, “Denshi Zairyo (Electronic Material),” 35 (10), 53(1996)).

[0003] Pattern formation methods used in the formation of such wiringand circuits include: a method which comprises the steps of: etching ametal, provided on a substrate having a layer construction ofmetal-insulating layer-metal, with an acidic solution, such as a ferricchloride solution, to form wirings, then subjecting the insulatinglayer, for example, to plasma etching, wet etching, or laser etching, toremove the insulating layer to form a desired shape, and connecting thewirings to each other, for example, through plating or electricallyconductive paste; and a method (Proceedings of the 7th Symposium ofJapan Institute of Electronics Packaging) which comprises the steps of:providing an insulating layer in a desired form using a photosensitivepolyimide (Japanese Patent Laid-Open No. 168441/1992) or the like; andthen plating gaps to form wiring.

[0004] A tendency toward downsizing of electric products in recent yearshas led to a reduction in thickness of each layer constituting metalconductor layer-polymeric insulating layer, and these layers each are inmany cases used in a thickness of not more than 100 μm. When wiring hasbeen formed of such thin layer, a warpage disadvantageously takes placein wiring due to a difference in coefficient of thermal expansionbetween the metal conductor layer and the polymeric insulating layer.Further, in the case of metal conductor layer-polymeric insulatinglayer-metal conductor layer, the formation of a circuit formationpattern or the like renders the area of the upper metal conductor layerdifferent from the area of the lower metal conductor layer, and, in thiscase, here again a warpage takes place in wiring.

[0005] When the thermal properties of the insulating layer and theconductor layer are known, the warpage of this substrate can becalculated according to the following equation (Miyaaki and Miki, NITTOTECHNICAL REPORT, 35 (3), 1 (1997)).$\sigma = {\frac{31E_{1}E_{2}}{2{h( {E_{1}^{2} + {14E_{1}E_{2}^{2}}} )}}\Delta \quad \alpha \quad \Delta \quad T}$

[0006] wherein

[0007] E1: modulus of the metal,

[0008] E2: modulus of the insulating layer,

[0009] Δα: difference in coefficient of thermal expansion between themetal and the insulating layer,

[0010] ΔT: temperature difference, and

[0011] h: layer thickness 1: wiring length.

[0012] According to this equation, the following two methods areconsidered effective for reducing the warpage of wiring:

[0013] 1. a reduction in modulus of insulating layer; and

[0014] 2. a reduction in the difference in coefficient of thermalexpansion between the insulating layer and the metal wiring layer.

[0015] Regarding the wiring formation method, in the substrate used inthe method for the formation of wiring through etching of a metal in thelaminate having layer construction of metal-insulating layer-metal or alayer construction of metal-insulating layer, in order to reduce thewarpage of the substrate, a low-expansion polyimide is used as theinsulating layer from the viewpoint of the necessity of rendering thecoefficient of thermal expansion of the metal identical to thecoefficient of thermal expansion of the insulating layer (U.S. Pat. No.4,543, 295, Japanese Patent Laid-Open Nos. 18426/1980 and 25267/1977).Since, however, the low-expansion polyimide is not generallythermoplastic, the adhesion to metals is poor making it difficult toprovide adhesive strength high enough to withstand practical use. Toovercome this problem, a thermoplastic polyimide resin or epoxy resinhaving good adhesion to the metal is used as an adhesive layer betweenthe metal and the low-expansion polyimide.

[0016] At the present time, rapid expansion of production of personalcomputers has lead to increased production of hard disks incorporated inthe personal computers. A component, in the hard disk, called a“suspension,” which supports a head for reading magnetism, is beingshifted in its main products from one, wherein copper wiring isconnected to a stainless steel plate spring, to one called a “wirelesssuspension” comprising copper wiring which has been connected directlyto a stainless steel plate spring, from the viewpoint of coping with thesize reduction.

[0017] The wireless suspension is mainly formed of a material having athree-layer structure. The material has a layer construction comprisingan insulating layer, a copper alloy foil provided on one side of theinsulating layer, and a stainless steel foil provided on the other sideof the insulating layer. Since scanning on a disk being rotated at ahigh speed is carried out, fine vibration is applied to the member.Therefore, the adhesive strength of the wiring is very important. Thisrequires satisfying severe specifications. The adhesive strength of thewiring depends greatly upon the material having a three-layer structurein its adhesive layer portion, and the ability of the adhesive layer assuch determines the adhesive strength as the product.

[0018] A polyimide or similar resin, which has good insulatingproperties even in a thin layer thickness, is used as the resin for theinsulating layer in the laminate having a layer construction ofmetal-insulating layer-metal or a layer construction of metal-insulatinglayer, particularly in the field of electronic members where long-termreliability is required. In order to impart adhesive properties to thepolyimide resin, it is common practice to impart thermoplasticity.However, there is few specific studies on the relationship between theadhesive strength of the polyimide resin and the properties ofadhesives. The present situation is, for example, such that, when theadhesive strength of the polyimide resin is examined, actual contactbonding followed by a peel test is necessary, that is, very troublesomework should be carried out.

DISCLOSURE OF THE INVENTION

[0019] Accordingly, it is an object of the present invention to providea laminate having a layer construction of metal-insulating layer-metalor a layer construction of metal-insulating layer, which laminate meetsconditions for developing large adhesive strength between the insulatinglayer and the metal, as well as to provide an insulating film and anelectronic circuit using the laminate.

[0020] In order to solve the above problems of the prior art, thepresent inventor has made extensive and intensive studies on theproperties and adhesive strength of the polyimide resin. As a result,the present inventor has found that the influence of the viscoelasticbehavior at the contact bonding temperature on the adhesive strength ismuch more significant than that of the composition of the resin on theadhesive strength. This has led to the completion of the presentinvention.

[0021] Maximizing the anchor effect created by biting of the resin intoconcaves and convexes on the surface of an adherend is consideredcontributable to enhanced adhesive strength. To this end, the contactbonding is preferably carried out at a temperature of Tg, in which thethermoplastic resin beings to develop fluidity, or above. In this case,however, the storage modulus at a temperature of Tg or above variesaccording to a difference in structure of the resin. Therefore, even inthe case of contact bonding at a temperature of Tg (glass transitionpoint) or above, the created adhesive strength varies depending upon thestructure of the resin. The present inventor has directed attention tostorage modulus as a measure of the fluidity of the resin and has madestudies on the relationship between the storage modulus and the adhesivestrength for thermoplastic resins having various different compositions.As a result, the present inventor has found a correlation such that theadhesive properties improve with lowering the storage modulus at atemperature at or above Tg. The present inventor has further found thatresins particularly having a storage modulus of not more than 10⁶ Pahave good adhesion to adherends independently of resin composition.

[0022] These demonstrate that the use of thermoplastic resins having astorage modulus of not more than 10⁶ Pa at a temperature at or above Tgcan provide laminates having good adhesive strength. More preferably,the storage modulus in the range of 10⁶ Pa to 10² Pa can providelaminates having better adhesive strength. The storage modulus of notless than 10⁶ Pa at a temperature at or above Tg is unfavorable. Thereason for this is that the fluidity is low at the time of contactbonding in the preparation of the laminate and, thus, the resin is lesslikely to bite into the concaves and convexes on the surface of theadherend and this makes it difficult to develop anchor effect. On theother hand, the storage modulus of not more than 10² Pa at a temperatureat or above Tg poses a problem such that, although the adhesive propertycan be exhibited, the fluidity of the adhesive layer is excessivelylarge and, consequently, the adhesive layer is squeezed out from thebonded surface in the step of contact bonding in the preparation of thelaminate.

[0023] Thus, according to one aspect of the present invention, there isprovided a laminate having a layer construction of first metallayer-insulating layer-second metal layer or a layer construction ofmetal layer-insulating layer, wherein

[0024] said insulating layer has a multilayer structure of two or morelayers,

[0025] the layer on the side of the adhesive interface with the metallayer, out of the layers constituting the insulating layer, is athermoplastic resin layer, and

[0026] the minimum value of the storage modulus at a temperature at orabove Tg of the thermoplastic resin layer is not more than 10⁶ Pa.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a diagram showing one embodiment of the layerconstruction of the laminate according to the present invention;

[0028]FIGS. 2A to 2D are diagrams showing one embodiment of a flow sheetof the production process of a laminate according to the presentinvention;

[0029]FIGS. 3A to 3D are diagrams showing another embodiment of a flowsheet of the production process of a laminate according to the presentinvention; and

[0030]FIG. 4 is a graph showing the relationship between the storagemodulus and the adhesive strength (g/cm) based on the results shown inTable 1, wherein the abscissa represents the storage modulus (Pa) andthe ordinate the adhesive strength.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] The present invention will be explained in detail.

[0032] The adhesive strength between the adhesive and the metal isminily determined by two factors, one of which is the affinity of themolecular structure of the adhesive for the surface of the metal and theformation of a bond with the metal and the other is the anchor effectattained by biting of the adhesive into concaves and convexes on thesurface of the metal.

[0033] The affinity for the surface of the metal embraces generallyconsiderable affinity and bond, such as chemical bond or coordinationbond to the surface of the metal and intermolecular interaction(interaction between atoms).

[0034] As a result of extensive and intensive studies, the presentinventor has found that the adhesive strength at the interface of themetal and adhesive bonded by contact bonding is influenced moresignificantly by the anchor effect than the bond between the molecularstructure and the metal and the affinity, and has also found therelationship between the properties of the fluidity of the adhesivelayer and the adhesive strength.

[0035] It is generally said that, when a resin having higher fluidity isused as an adhesive, the anchor effect is more likely to occur. In fact,however, there is no report about detailed studies on the fluidity ofthe adhesive layer, and the principle of the present invention is veryuseful for the preparation of laminates having good adhesive strength.

[0036] Metals or films as the adherend are not particularly limited.However, adherends having certain concaves and convexes formed, forexample, by hydrophobilization of the surface are likely to developadhesive strength by the anchor effect. In this case, it should be notedthat when the thickness of the adhesive layer is lower than the heightof the concaves and convexes of the adherend, a space is formed betweenthe adherend and the adhesive layer, leading to lowered adhesivestrength.

[0037] Thermoplastic Resin

[0038] According to the present invention, the insulating layer on theside of the adhesive interface with the metal layer is formed of athermoplastic resin. Polyimides, which have low coefficient of thermalexpansion and are highly heat resistant, or resins having propertiessimilar to the polyimides are preferred as the insulating layer from theviewpoint of the necessity of rendering the coefficient of thermalexpansion of the metal identical to that of the insulating layer. Theterm “thermoplastic resin” as used herein refers to resins having aclear glass transition point. The resin, however, is not particularlylimited and is independently of the presence or absence of the imidebond, so far as the resin has heat resistance and insulating property.Thermoplastic resins preferably usable in the present invention include,but are not particularly limited to, resins having an imide bond in themolecule thereof, such as polyimides, polyamide-imides,polyether-imides, and maleimide-modified resins, and resins having arelatively high glass transition point, such as aromatic polyesters andaromatic polyamides.

[0039] There is a tendency of a correlation such that the adhesivestrength increases with lowering the storage modulus at a temperature ator above Tg of the thermoplastic resin. The preparation of resins so asto lower the storage modulus generally results in a tendency toward theformation of thermoplastic resins having lower Tg.

[0040] The term “storage modulus” as used herein refers to the storagemodulus of the thermoplastic resin as the adhesive at the time ofbonding between the insulating layer and the adherend, for example, bycontact bonding. In this connection, it should be noted that, in somecases, the state of the material having a three layer structure as thefinal form is different from the state of the bonding step, for example,in molecular structure due to heat history. Therefore, the storagemodulus does not refer to the storage modulus in the changed state.

[0041] In general, the weight average molecular weight of thethermoplastic resin according to the present invention is preferably6000 to 500000, particularly preferably 8000 to 80000, although theweight average molecular weight varies depending upon the molecularstructure. When the molecular weight is not less than 500000, it isdifficult to provide homogeneous coating. Further, the larger thestorage modulus at a temperature at or above Tg, the lower the fluidityand the lower the tendency of the attainment of the anchor effect. Ingeneral, in the case of resins having the same chemical composition, thelower the molecular weight, the lower the Tg (glass transition point)and the lower the storage modulus at a temperature at or above Tg. Whenthe molecular weight is not more than 6000, the film forming property ispoor making it difficult to provide a homogeneous coating of athermoplastic resin layer.

[0042] The thermoplastic resin as the adhesive may be coated in asolution form, or alternatively may be applied by a different method,for example, in a film form. Further, a method may also be used whichcomprises applying a precursor or a derivative of the thermoplasticresin, performing molding and then processing the molded product toconvert the chemical structure to a desired chemical structure.

[0043] Laminate

[0044]FIG. 1 shows one embodiment of the layer construction of thelaminate according to the present invention. Numeral 1 designates aninsulating layer. A thermoplastic resin layer 2 is stacked on both sidesof the insulating layer 1. A copper foil 3 or an alloy foil is stackedas a metal layer on one of the thermoplastic resin layers 2, and an SUSfoil (a stainless steel foil) 4 is stacked as a metal layer on the otherthermoplastic resin layer 2.

[0045] At least one layer constititing the insulating layer may beformed of a polyimide resin or may be a polyimide film. Alternatively,all the layers constituting the insulating layer may be formed of apolyimide resin or may be a polyimide film.

[0046] Each metal layer is preferably formed of a material selected fromthe group consisting of copper alloy, copper, and stainless steel. Thefirst metal layer may be formed of a material which is the same as ordifferent from that constituting the second metal layer.

[0047] Insulating Film

[0048] An insulating film comprising a resin film or a resin layer as aninsulating layer and a thermoplastic resin layer, having a minimum valueof storage modulus of not more than 10⁶ Pa at a temperature at or aboveTg, provided on both sides or one side of the insulating layer may beused as an intermediate material for the production of the laminateaccording to the present invention. At least one resin layerconstituting the insulating layer may be a polyimide film or may beformed of a polyimide resin. Alternatively, all the layers constitutingthe insulating layer may be formed of a polyimide resin or may be apolyimide film.

[0049] Production Process of Laminate

[0050] A film coating process and a metal coating process will bedescribed as a production process of a laminate having layerconstruction of first metal layer-insulating layer-second metal layer asan example of the laminate composed of a metal layer and an insulatinglayer. The production process is not particularly limited to these only.The production process of the laminate according to the presentinvention will be described by taking, as an example, the use of apolyimide film as an insulating layer.

[0051] 1) Film Coating Process

[0052]FIG. 2 is an embodiment of a flow sheet showing a productionprocess of the laminate according to the present invention. According tothis embodiment, as shown in FIGS. 2A to 2D, a polyimide film isprovided as an insulating layer 1 (FIG. 2A). A thermoplastic polyimidesolution is coated on both sides of the polyimide film, and the coatedpolyimide film is dried to remove the solvent to form a thermoplasticresin layer 2 as an adhesive layer (FIG. 2B). Next, as shown in FIG. 2C,a copper foil 3 and an SUS foil (a stainless steel foil) 4 are broughtinto intimate contact with respective both sides of the polyimide filmthrough the respective thermoplastic resin layers 2, that is, the copperfoil 3 is formed on one of the thermoplastic resin layers 2, while theSUS foil 4 is formed on the other thermoplastic resin layer 2.Thereafter, as shown in FIG. 2D, thermocompression bonding is carriedout at a temperature at or above the softening point (Tg) of polyimidein the thermoplastic resin layer 2 while applying high pressure.

[0053] 2) Metal Coating Process

[0054]FIG. 3 is another embodiment of a flow sheet showing a productionprocess of the laminate according to the present invention. As shown inFIGS. 3A to 3D, a copper foil 3 and an SUS foil (a stainless steel foil)4 are provided (FIG. 3A). A polyimide solution is coated on one side ofeach of the copper foil 3 and the SUS foil 4, and the coated copper foil3 and the coated SUS foil 4 are dried to remove the solvent to form athermoplastic resin layer 2 on the copper foil 3 and on the SUS foil 4(FIG. 3B). A polyimide film as an insulating layer 1 is sandwichedbetween the copper foil 3 with the thermoplastic resin layer 2 formedthereon and the SUS foil (stainless steel foil) 4 with the thermoplasticresin layer 2 formed thereon so that the thermoplastic resin layers 2face each other, followed by intimate contact (FIG. 3C). Thereafter,thermocompression bonding is carried out at a temperature at or abovethe softening point (Tg) of the thermoplastic resin layer 2 whileapplying high pressure (FIG. 3D).

[0055] For each coating process, the thermocompression bonding ispreferably carried out at a temperature such that exhibits the minimumvalue of the storage modulus of the thermoplastic resin. This is becausethe fluidity of the thermoplastic resin is best and the anchor effectattained by biting of the thermoplastic resin into the concaves andconvexes on the surface of the adherend is maximized to enhance theadhesive strength between the metal layer and the core insulating layer.

[0056] Electronic Circuit

[0057] An electronic circuit can be generally formed by the followingmethod.

[0058] At the outset, a photosensitive resin layer is coated orlaminated on the surface of a metal on its side where the formation of acircuit is desired. A mask with a desired pattern image formed thereonis brought into intimate contact with the photosensitive resin layer,followed by exposure to an electromagnetic wave with wavelength to whichthe photosensitive resin is sensitive. Thereafter, when thephotosensitive resin is of a positive-working type, the exposed area isdeveloped with a predetermined developing solution. On the other hand,when the photosensitive resin is of a negative-working type, theunexposed area is eluted with a predetermined developing solution. Thus,a desired circuit image is formed on the metal. The metal with thecircuit image formed thereon is then immersed in a solution capable ofdissolving the metal, such as an aqueous ferric chloride solution.Alternatively, this solution may be sprayed on the substrate. Thus, themetal exposed on the surface is eluted, and the photosensitive resin isthen peeled off by a predetermined peeling solution to form a circuit.

[0059] When etching of the insulating layer is necessary, a desiredpattern may be formed on the circuit prepared in the above manner,followed by patterning of the insulating layer by a dry or wet process.

EXAMPLES

[0060] Dynamic Viscoelastic Test

[0061] Resins, i.e., polyamic acid varnish [PAA-A (tradename)manufactured by Mitsui Chemicals Inc.] as a precursor-type polyimide;polyamide-imide varnish [N 8020 (tradename) manufactured by Toyobo Co.,Ltd.] as a polyamide-imide; and polyimide varnish [SN-20 (tradename),PN-20 (tradename), and EN-20 (tradename), manufactured by New JapanChemical Co., Ltd.] as solvent-soluble, ring-closing-type polyimide,were used in a dynamic viscoelastic test. Metal foils, i.e., a rolledcopper foil [18 μm (layer thickness), RCF-T5B (tradename) manufacturedby FUKUDA METAL FOIL & POWDER CORPORATION] and a stainless steel foil[20 μm (layer thickness), SUS 304 H-TA foil (tradename) manufactured byNippon Steel Corp.], were provided as a substrate and used in anadhesive property test. Further, a polyimide film [75 μm (layerthickness), APIKAL NPI film (tradename) manufactured by KanegafuchiChemical Ind. Co., Ltd.] was used for studies on adhesion to the resins.

[0062] Each resin was coated on each substrate having a size of 10 cm×10cm and a layer thickness of 12 μm, and all the coated substrates exceptfor PAA-A (tradename) were dried in an oven at 180° C. for 30 min. ForPAA-A (tradename) which is an amic acid varnish, the solvent was removedby drying at 120° C. for 15 min, and the coated substrate was thensubjected to a predetermined procedure to perform thermal imidation,thereby preparing a polyimide. After the formation of the coating in athickness of about 20 μm, etching of the substrates was carried out in45 Baume ferric chloride having a liquid temperature of 50° C. toprepare coating substrates. These coating substrates were taken off toobtain test pieces having a size of about 1.5 cm in length×5 mm inwidth. These coating substrates were measured for storage modulus ateach temperature by means of a viscoelastic measuring apparatus RSA-II(tradename) manufactured by Rheometric Scientific under conditions ofsample length 8 mm, sample width 5 mm, temperature rise rate 5° C./min,frequency 3.0 Hz, and temperature rise from room temperature to 400° C.

[0063] Evaluation of Adhesive Property

[0064] Concaves and convexes were intentionally provided on the surfaceof the substrate so that separation does not take place between theadhesive layer and the substrate and interfacial peeling between theadherend and the adhesive layer or cohesive failure of the adhesivelayer necessarily takes place. The surface of a 100 μm-thick SUS 304plate was roughened by means of a wet blasting machine manufactured byMACOHO using #1000 alumina as an abrasive under conditions of pressure0.7 kg/cm² and scanning speed 10 mm/sec, and the surface was thenultrasonically washed with pure water for 30 min to remove the abrasivedeposited on the surface. In this case, both sides of the plate wereroughened because roughening of only one side causes warpage of the SUS304 plate. Thereafter, a 2 to 3 μm-thick coating was spin coated on thesurface, and the coating was dried or imidated under the above-describedconsitions to form an adhesive layer on the SUS 304 plate. Desiredmetals and films were stacked on the assemblies, followed by vacuumcontact bonding at a temperature, which renders the storage modulus ofeach sample lowest, at a surface pressure of 1 MPa for 10 min to preapesamples. Here the peak of Tanδ as obtained from the measurement ofviscoelasticity was regarded as Tg.

[0065] The samples were cut with a hand push cutter into a size of 1 cmin width, followed by a 90-degree peel test at a tensile speed of 500mm/min by means of a material tester (type 5565) manufactured byInstron. The test results on the adhesive property, Tg, and the lowestvalue of the storage modulus are shown in Table 1. TABLE 1 Min. value ofstorage modulus Substrate Tg at temp. of Tg or above RCF-TSB SUS 304H-TA APIKAL NPI Thermo- PAA-A 205° C. 1.2 × 10⁴ Pa 1250 g/cm 1300 g/cm 920 g/cm plastic N 8020 315° C. 1.0 × 10⁸ Pa  190 g/cm  10 g/cm  270g/cm resin SN-20 305° C. 1.0 × 10⁷ Pa  250 g/cm  70 g/cm  200 g/cm PN-20285° C. 1.5 × 10⁶ Pa  300 g/cm  200 g/cm  360 g/cm EN-20 160° C. 1.0 ×10⁴ Pa 1250 g/cm  820 g/cm 1600 g/cm

[0066]FIG. 4 is a graph showing the relationship between the storagemodulus and the adhesive strength based on the results shown in Table 1,wherein the abscissa represents the storage modulus (Pa) and theordinate the adhesive strength. In FIG. 4, ♦ represents data on RCF-T5Bfoil (tradename) manufactured by FUKUDA METAL FOIL & POWDER CORPORATION,▪ represents data on SUS 304 H-TA foil (tradename) manufactured byNippon Steel Corp., and ▴ represents data on APIKAL NPI film (tradename)manufactured by Kanegafuchi Chemical Ind. Co., Ltd. As is apparent fromthe graph shown in FIG. 4, lower storage modulus at temperatures of Tgor above provides better adhesion to each substrate.

[0067] Production of laminate

[0068] An APIKAL NPI film (tradename, manufactured by KanegafuchiChemical Ind. Co., Ltd.) having a thickness of 12.5 μm (layer thickness)was applied as a polyimide core film to a 100 μm thick SUS 304 plate. Athermoplastic polyimide varnish EN-20 manufactured by New Japan ChemicalCo., Ltd. was spin coated on one side of the polyimide film applied ontothe substrate to a final layer thickness of about 2 μm, and the coatingwas dried at 180° C. for 30 min in the air to remove the solvent.Thereafter, the film was separated from the substrate, and the substratewas turned over. The film was again applied to the substrate, and anadhesive layer was formed in the same manner as described above. Thefilm with an adhesive layer formed on both sides thereof was sandwichedbetween a 20 μm thick SUS 304 HTA and a rolled copper foil RCF-T5Bmanufactured by FUKUDA METAL FOIL & POWDER CORPORATION (thickness 18μm), and vacuum contact bonding was carried out under conditions of 300°C., 1 MPa, and 10 min.

[0069] For various samples, laminates were prepared in the same manneras described above, and circuits were formed by the following method.The temperature, at which contact bonding to the laminate was carriedout, was the temperature at which the lowest storage modulus wasprovided in the dynamic viscoelasticity test, and the pressure and thetime were 1 MPa and 10 min for all the cases. For the polyamic acidvarnish [PAA-A (tradename) manufactured by Mitsui Chemicals Inc.] whichis a precursor varnish, a tack-free precursor layer was formed on bothsides of the film, and the polyamic acid varnish on both sides of thefilm was simultaneously thermally imidated by a predetermined method toprepare a film provided with an adhesive layer, followed by stacking.

[0070] Circuits were prepared as follows. An assembly composed ofSUNFORT AQ 5038 (a negative-working dry film manufactured by AsahiChemical Industry Co, Ltd.) laminated onto the copper side of thethree-layer material was exposed by a contact exposure system through apredetermined mask. The exposed assembly was developed with a 1% aqueoussodium carbonate solution, was immersed in a 45 Baume aqueous ferricchloride solution to remove the exposed copper, and was then immersed ina 3% aqueous sodium hydroxide solution of 50° C. for one min to removethe dry film.

[0071] The circuit prepared from the three-layer material had a desiredshape.

[0072] According to the present invention, the interposition, as anadhesive layer, of a thermoplastic resin having a minimum value ofstorage modulus of not more than 10⁶ Pa at a temperature at or above Tgof the thermoplastic resin layer at the interface between the metallayer and the insulating layer can provide laminates having goodadhesive strength between the metal layer and the insulating layer.

1-10. (Canceled).
 11. A process for producing a laminate, comprising thestep of thermocompression bonding a core insulating layer, athermoplastic resin layer, which is disposed on both sides or one side(z-plane) of the core insulating layer, has an adhesive property and hasa minimum value of the storage modulus of not more than 10⁶ Pa at atemperature at or above Tg of the thermoplastic resin layer, and a metallayer disposed on the surface of the thermoplastic resin layer to oneanother at a temperature of Tg or above.
 12. The process for producingthe laminate according to claim 11, wherein the thermocompressionbonding is carried out under temperature conditions such that thestorage modulus of the thermoplastic resin is minimum.
 13. A method forproducing an electronic circuit comprising the steps of: providing alaminate comprising a combination of a metal layer with an insulatinglayer, said laminate having a layer construction of first metallayer/insulating layer/second metal layer or a layer construction ofmetal layer/insulating layer, the insulating layer having a multilayerstructure of two or more layers, the layer on the side of the adhesiveinterface with the metal layer, out of the layers constituting theinsulating layer, being a thermoplastic resin layer, and a maximum valueof the storage modulus at a temperature at or above Tg of thethermoplastic resin layer being not more than 10⁶ Pa; forming aphotosensitive resin layer on a surface of the metal layer of thelaminate; and patterning the thus formed photosensitive resin layer toprepare an electronic circuit.
 14. A method for producing an electroniccircuit comprising the steps of: providing an insulating film comprisingan insulating layer and a thermoplastic resin layer provided on at leastone side of the insulating layer, the thermoplastic resin having amaximum value of the storage modulus of not more than 10⁶ Pa at atemperature at or above Tg of the thermoplastic resin layer; laminatingthe insulating film with a metal layer to prepare a laminate; forming aphotosensitive resin layer on a surface of the metal layer of thelaminate; and patterning the thus formed photosensitive resin layer toprepare an electronic circuit.